1. Field of the Invention
The present invention relates generally to gel compounds within conduits or buffer tubes and more specifically to the reduction of dripping of the gel compounds at higher temperatures.
2. Description of Related Art
Fiber optic cables have been used by the telecommunications industry for a number of years to transmit information at very high rates over long distances. Fiber optic cables come in a variety of configurations, including: cables with a centrally located single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside.
The buffer tubes within the ribbon cable generally contain one or more fiber optic ribbons centrally located within the buffer tube and a gel compound surrounding the optical fiber ribbons. An example of this can be seen in FIGS. 1–4. As shown in these figures, the fiber optic ribbons 3 are centrally located within buffer tube 1. As can be further seen from FIGS. 1–4, a gel compound 2 surrounds the fiber optic ribbons 3. The gel compound 2 serves a number of purposes. One purpose is to provide a cushioning media between the buffer tube 1 and the fiber optic ribbons 3 to thereby prevent the fiber optic ribbons 3 from contacting the sides of the buffer tube 1. The cushioning media dissipates radial crushing force and in addition, the gel compound 2 provides delayed motion response to the fibers under scanning bending loads. Such loads occur during the installation, when cables are pulled around the corners of the ducts or over the sheaves. The same applies to the earlier stages of manufacture when buffer tube 1 is bent over the sheaves and radially compressed by caterpillars. The artificial increase in the inertia of the ribbons 3 is provided by the viscous gel media and results in time delay for fibers to accommodate the load and to move slower than in a non-gel media toward the tube walls 1. When the fiber optic ribbons 3 contact the sides of the buffer tube 1, signal attenuation problems occur due to micro-bending and high stress. The gel compound 2 also serves to prevent exterior items from coming into contact with the fiber optic ribbons 3 if the buffer tube 1 is penetrated. For example, the gel compound 2 protects the fiber optic ribbons 3 from water that might penetrate the buffer tube 1.
Several problems occur in these conventional buffer tubes, especially ones in which the buffer tube 1 diameter is large (for example, greater than 0.310 inches). First, when the temperature of the gel compound 2 increases, the viscosity and yield stress of the gel compound 2 decreases. If the yield stress of the gel decreases below a critical value, the gel compound 2 may begin to flow. For example, if the buffer tube 1 is physically positioned in a vertical manner, as shown in FIG. 5, and the buffer tube 1 is heated, the gel compound 2 within the buffer tube 1 may begin to flow towards the bottom of the buffer tube 1, leaving a cavity 4.
In more detail, as the temperature of the buffer tube 1 increases, the buffer tube 1 expands, thereby increasing the diameter and length of the buffer tube 1, according to the difference between the coefficient of thermal expansion (“CTE”) of the buffer tube material 1 and gel compound 2. As for the gel compound 2, as noted above, as its temperature increases, the viscosity and yield stress of the gel compound 2 decreases. As shown in FIG. 5, gravity provides a downward force to the gel compound 2 while frictional forces (F1 and F2) with the tube wall are transmitted through the material by the yield stress of the gel compound 2. Friction between the gel compound 2 and the buffer tube 1 is labeled F1 while the fiction between the gel compound 2 and the fiber optic ribbons 3 is labeled F2. Consequently, as the temperature of the gel compound 2 increases, the yield stress of the gel compound 2 decreases and the ability of the gel to transmit friction forces F1 and F1 through the gel compound 2 decreases. Since the downward force of gravity remains constant during an increase in temperature of the gel compound 2, when the temperature of the gel compound 2 increases, the downward force of gravity on the material becomes greater than the upward force that can be transmitted through the material through the yield stress of the gel compound 2. As a result, the gel compound 2 may flow downward.
Additionally, gel compound 2 may be “forced” out of the buffer tube 1 when heated due to the difference between the CTE of the buffer tube 1 and the CTE of the gel compound 2. As stated earlier, when heated, both the buffer tube 1 and the gel compound 2 expand according to their respective CTE. If the CTE of the buffer tube 1 is less than the CTE of the gel compound 2, then the gel compound 2 expands more than the buffer tube 1. Since the gel compound 2 is expansionally limited in the radial direction by the buffer tube 1, if the gel compound 2 expands more than the buffer tube 1 when heated, the additional expansion of the gel compound 2 is directed in the axial direction. As a result, gel compound 2 is “forced” out of the ends of the buffer tube 1.
Once the gel compound 2 flows out of the buffer tube 1, it does not provide adequate protection to the fiber optic ribbons 3. The fiber optic ribbons 3 tend to contact the buffer tube walls 1, which in turn may cause attenuation problems. Additionally, gel compound 2 flowing out of the buffer tubes 1 will flow into splice enclosures which make later access to the closures problematic. Therefore, it is an object of the present invention to improve the compound flow performance of gel-filled fiber optic cables.